This invention relates to a control valve for a construction equipment, and more particularly to a control valve adapted to cause fluid to be supplied from a separate circuit system to a boom cylinder of a construction equipment to increase a speed of the cylinder.
Conventionally, a construction equipment has generally employed a system in which a plurality of control valves are connected to each of a pair of circuit systems. Each of the control valves connected to one of the circuit system is generally constructed in such a manner as shown in FIGS. 1 and 2.
More particularly, the control valve includes a first spool 2 provided in a valve body a so as to operate a central by-pass passage 1 and a second spool 3 arranged on a downstream side of the first spool 2 so as to carry out speeding-up and control of a boom cylinder.
The first spool 2 is so arranged that one end thereof faces a pilot chamber 4 and the other end thereof faces a spring chamber 5. Also, the first spool 2 is adapted to open the central by-pass passage 1 through an annular groove 6 when it is at its normal position shown in FIG. 2. When a pilot pressure is applied to the pilot chamber 4 to move the spool 2 against a spring 7, the central by-pass passage 1 is closed.
The second spool 3 has one end facing a pilot chamber 8 and the other end facing a spring chamber 9. When the second spool 3 is at its normal position shown in FIG. 2, the central by-pass passage is opened through a first annular groove 10 and a parallel passage 11 communicating with the boom cylinder (not shown) is closed. However, when a pilot pressure is applied to the pilot chamber 8 to move the spool 3 against a spring 12, the central by-pass passage 1 is closed and the parallel passage 11 is opened through a second annular groove 13.
When each of the spools 2 and 3 is held at the normal position shown in FIG. 1, the parallel passage 11 is closed and the central by-pass passage 1 is opened, so that fluid discharged from a pump (not shown) is supplied through an orifice 14 to a tank T. When fluid thus flows to the orifice 14, pressure loss occurs at the orifice 14 to generate a pressure on an upstream side of the orifice 14. The control valve causes the pressure produced on the upstream side to act on a control mechanism of a variable discharge pump (not shown) through a passage 15, resulting in decreasing a discharge rate of the discharge pump.
Then, when a pilot pressure is applied to the pilot chamber 4 of the first spool 2 to close the central by-pass passage 1, pressure loss through the orifice 14 is avoided, so that a pressure acting on the control mechanism of the variable discharge pump is decreased to zero to cause the discharge rate of the discharge pump to be maximized. An actuator connected to the circuit system is actuated by maximizing the discharge rate of the variable discharge pump.
When the second spool 3 is changed over irrespective of a switching position of the first spool 1, the central by-pass passage 1 is closed. This causes the discharge rate of the variable discharge pump to be maximized and fluid discharged from the pump to be supplied to the parallel passage 11. The fluid thus supplied to the parallel passage 11 is then supplied to the other circuit system.
The conventional control valve described above is required to provide the valve body a with the first spool 2 and the second spool 3 separately, so that it is required to form two spool holes in order to arrange the spools in the valve body. This causes manufacturing of the control valve to be highly troublesome. Also, this requires a large space to be formed in the valve for receiving the spools therein, resulting in complicating a structure of the valve and large-sizing the valve.